In many applications a voltage regulator is required to provide a voltage within a predetermined range. Some circuits are subject to uncertain and undesirable functioning and even irreparable damage if an input power supply falls outside a certain range.
AC to DC converters are typically designed to operate over a wide voltage range such as 90 VAC to 264 VAC so that the converters can operate with AC line voltages from anywhere in the world. In practice, the AC line voltages encountered throughout the world lie within the ranges 90 VAC-132 VAC for “low-line” operation and 180 VAC to 264 VAC for “high-line” operation. The cost of the main switching semiconductors of conventional AC to DC converters and the losses in the converters increase dramatically with the range of input voltages supported by the converter. This is especially true when the converters use MOSFETs for the main switching semiconductors. A converter designed to operate over only one of the “low-line” or the “high-line” range, a ratio of 1.46:1, would be less costly and more efficient than a converter designed to operate over the entire range of “low-line” and “high-line”, 90 VAC to 264 VAC, a ratio of 2.93:1.
Conventional low-power converters, typically less than 65 watts, usually do not have government requirements for a high power factor and typically operate with a low power factor. These low-power converters usually employ voltage doublers that are enabled at “low-line” and disable at “high-line.” The converter circuit following the voltage doubler is designed with circuits rated for “high-line” operation, even if the AC line is at “low-line”. This is inefficient and costly.
FIG. 5 illustrates the conventional line switcher 500. In the conventional line switcher 500, at approximately 230 VAC of an AC line 502, the switch S1 is open so that a standard diode bridge 505 charges a first and second large electrolytic capacitors C1, C2 that are in series to the peak of the 230 VAC of the AC line 502. The switch S1 can be a triac, a mechanical switch, or can be a configuration of contacts that occurs when a user inserts a line plug connector block into a device. At 115 VAC of the AC line 502, the switch S1 is closed causing each of the first and second large electrolytic capacitors C1, C2 to charge to the peak of the 115 VAC of the AC line 502. Since the capacitors C1 and C2 are in series, the total voltage Vin is a value that is twice the peak voltage of the AC line 502. Effectively, the switch S1 causes an input section of the line switcher 500 to operate as a voltage doubler at low-line.
The switch S1 is controlled to couple a terminal of the AC line 502 to a tie point 514 of the first and second large electrolytic capacitors C1, C2. At high-line, both the first and second large electrolytic capacitors C1, C2 are connected across the diode bridge 505 during both positive and negative half cycles. At low-line, only one of the first and second large electrolytic capacitors C1, C2 is connected across the diode bridge 505 at a time. Specifically, the capacitor C1 is connected during the positive half-cycle and the capacitor C2 is connected during the negative half cycle. The first and second large electrolytic capacitors C1, C2 have a capacitance so large that the first and second large electrolytic capacitors C1, C2 are able to maintain their charge from one 50/60 Hz cycle to the next cycle.
In the conventional line switcher 500, a single converter (not shown) can be connected across the series combination of the first and second large electrolytic capacitors C1, C2 (Cin). An advantage is that the single converter (not shown) is always driven at a voltage equal to the peak of the voltage at high-line, regardless of whether the input AC voltage of the AC line 502 is at high-line or low-line. The converter cost is therefore decreased. A disadvantage of the conventional line switcher 500 is that it draws a power with a poor power factor and is therefore only useful, in practice, for small converters such as those below 75 W. Another disadvantage of the conventional line switcher 500 is a reduced efficiency caused by the high pulsed currents used to charge the first and second large electrolytic capacitors C1, C2.
Accordingly, it is desirable to create a regulated power supply operable over both “low-line” and “high-line” ranges with a reduced cost and a greatly increased power factor and efficiency.